1. Field of the Invention
The present invention relates to interrogating an optical fiber to obtain information about the fiber or devices in communication with the fiber. More particularly, the interrogating is performed using optical reflectometry in a borehole penetrating the earth.
2. Description of the Related Art
In exploration and production of hydrocarbons, it is often necessary to drill a borehole into the earth to gain access to the hydrocarbons. Equipment and structures, such as borehole casings for example, are generally disposed into a borehole as part of the exploration and production. Unfortunately, the environment presented deep into the borehole can place extreme demands upon the equipment and structures disposed therein. For example, the equipment and structures can be exposed to high temperatures, pressures, and vibrations that can effect their operation and longevity.
In order to monitor the health of the equipment and structures disposed downhole, a fiber-optic distributed sensing system (DSS) may be used. Sensing fiber (an optical fiber containing sensors or in itself functioning as a sensor) or sensors may be attached to the equipment and structures at various locations usually at different depths in the borehole. The sensors can measure temperature, pressure, strain, and other parameters. By measuring strain for example, the system can determine if borehole casing is being deformed.
In one type of DSS, swept-wavelength (of light) interferometry can be used to interrogate a series of fiber Bragg gratings. Each fiber Bragg grating (FBG) in the series acts as a sensor. The optical fiber, in one example, is affixed to casing or wrapped along a length of the casing. As each FBG is exposed to a changing condition, the optical characteristics of each FBG will change in relation to the changed condition. A sensor interrogator is used to measure the optical characteristics of each of the FBGs in order to ascertain the changing conditions.
With conventional swept-wavelength Optical Frequency Domain Reflectometry (OFDR), also referred to as coherent OFDR, a swept-wavelength light source is coupled to an optical fiber. The optical fiber includes a reference reflector and a series of FBGs. The wavelength of light from the light source is swept to interrogate each of the FBGs. The reference reflector forms an interferometric cavity, such as a Fabry-Perot cavity, with each individual FBG.
As the wavelength of light from the light source is swept, an interferogram is created with a frequency for each interferometric cavity that is proportional to the length of the cavity for each FBG. Thus, spectral data from each FBG is modulated with a unique frequency, which ultimately permits individual inspection of the FBGs through conventional signal processing techniques. Converting the spectral data into the spatial frequency domain through a Fast Fourier Transform yields a view of the fiber with one set of measurements that includes the amplitude of the reflected light as a function of distance, where the distance is derived from the time. In this manner, each FBG can be monitored and treated as an individual sensor.
Unfortunately, the same vibration that can adversely affect the equipment disposed in a borehole can also adversely affect the conventional coherent OFDR system. The vibrations can affect light signals in the optical fiber, thereby, limiting the usable length of the optical fiber. The longer fiber lengths especially provide more opportunity for light interaction with the vibration, resulting generally, in increased distortion with longer lengths.
The conventional coherent OFDR system can also have problems relating to nonlinear tuning of the wavelengths of light transmitted into the fiber. These nonlinearities can reduce the fidelity of the measurements.
Therefore, what are needed are techniques to interrogate an optical fiber that reduce susceptibility to vibration of the optical fiber or to the tuning rate nonlinearities of the light used for the interrogation.